MILITARY snipers are competitive types. There is an ongoing and grisly contest between them to see who can kill an enemy soldier from the farthest distance away.

The present record is held by Craig Harrison, a corporal in the British Army's Household Cavalry, who managed to kill two Taliban soldiers from 2,475 metres in November 2009.

That was a slight improvement on the previous record, held by Rob Furlong, a Canadian soldier also fighting in Afghanistan, who managed to shoot his enemy in the chest from a distance of 2,473 metres in 2002.

Such long-range killings are the exception rather than the norm, as long-distance shooting is extremely difficult. Snipers must guess at wind direction, atmospheric density, relative humidity and a host of other factors that affect a bullet's trajectory. In an interview with the Sunday Times, Corporal Harrison described the atmospheric conditions as perfect: "no wind, mild weather, clear visibility", but modestly neglected to mention the hundreds of hours of painstaking training that are required on top of that.

Now a protoype self-aiming bullet developed at Sandia National Laboratories, an American weapons-research lab in New Mexico, might allow any soldier to match such feats. Modern bullets gain their accuracy from a technique known as rifling, whereby the barrel of the gun that fires them has a series of spiralling grooves etched into it. These cause the bullets to spin, and that spins stabilises their flight path. Rifling offers a huge improvement over old-style smoothbore guns such as muskets, which were notoriously inaccurate at even comparatively short ranges.

Sandia's researchers, though, have plumped for an old-style smoothbore barrel. That is because, instead of spin, their bullet is stabilised by four steerable fins at its rear. Those fins are linked to a computer chip that is, in turn, linked to an optical sensor on the bullet's nose. A laser is shone at the intended target, the bullet is fired, and the chip uses the fins to adjust the bullet's trajectory in mid-flight, a system similar in principle to the one used on anti-aircraft missiles.

The researchers say that computer simulations suggest that, at a range of half a mile, a typical unguided bullet would miss a target by an average margin of nine metres or so, but that their guided bullet could cut that to just 20 centimetres. And a quirk of ballistics means that, at longer ranges, the system's accuracy should get better. As it flies, the bullet pitches and yaws through the air, and that natural rhythym limits the frequency with which its on-board computer can make course corrections. But the longer the bullet is in flight, the less violent those movements become, allowing increasingly precise tracking.

So far, the bullet is only a prototype, and Sandia (which is a government lab managed by Lockheed Martin, a giant American weapons firm) is looking to join forces with other firms to bring the bullet to market. If they are successful, Corporal Harrison's record may not stand for long.